chapter 7: durability and service life - wikiplast

7Chapter

Durability

and

Service Life

Lester H. Gabriel, Ph.D., P.E.

WE TAKE CARE ABOUT THE FUTURE

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CHAPTER 7: DURABILITY AND SERVICE LIFE

Durability of Drainage Pipes

Durability is the property to resist erosion, material degradation and subsequent loss of function due to environmental and/or other service conditions. Abrasion, chemicalcorrosion and electrochemical corrosion are the most common durability concerns for drainage pipes. Erosion of drainage pipes by changes in �ow patterns also mayinclude: impingement by suspended solid particles or gas bubbles striking the surface;turbulence at pipe entrances and sharp bends, as well as aggregate and sedimentdeposits. Although an unlikely event for culverts and storm drains, high pressureand sub-atmospheric pressures that may be associated with high velocity �ows maycause cavitation.

Corrosive chemicals carried by the water expose the inverts of storm drain pipelinesand culverts to corrosion-abrasion damage. The invert, host to both an electrolyte and varying concentrations of oxygen, may also be exposed to electrolytic corrosion.

In hostile environments, materials such as unprotected concrete and unprotected steeldevelop corrosion products that are more brittle and thus more vulnerable to bedloadabrasion. As the corroded surface is stripped away, a fresh surface is exposed and newcorrosion products form. If this cycle continues, eventual structural failure must be considered. Longevity of exposed pipes depends upon the qualities of the protectivebarriers. Palliative measures – such as protective coatings, linings and pavements – are at risk of being eroded, cracked or delaminated.

Corrosion

Chemical corrosion of buried pipelines and culverts may occur in the presence of soils and waters containing acids, alkalis, dissolved salts and organic industrial wastes.Surface water, ground water, sanitary e�uent, acid rain, marine environments andmine drainage carry these contaminants. Some may occur in regions of high rainfall,others in arid locations. Sulfates, carbonates and chlorides degrade concrete – aprocess often accelerated in regions where freeze-thaw cycles leave the material opento deeper penetration by the o�ending elements. Vitri�ed clay and plastic pipes arelargely inert. Zinc, aluminum, aluminum-zinc alloy metallic coatings, asphaltic coat-ings with and without �ber and polymer coatings o�er metal pipes varying measuresof protection against soil-side and water-side chemical and electrochemical corrosion.

DURABILITY AND SERVICE LIFE




Electrochemical corrosion of metal pipelines and culverts may occur where oxygenstarved and oxygen rich locations on, and in the vicinity of, the pipe respectivelybecome anodes and cathodes. A potential di�erence will cause current �ow through a circuit composed of an electrolyte (soil moisture in the vicinity of the pipe or liquid within), an anode (a region on the pipe giving up electrons), a cathode (aregion on the pipe accepting electrons) and the pipe as a conductor. Loss of pipematerial occurs at the anode. Stray direct current from a nearby electric railway or acathodically protected utility is another source of potential di�erence. The degree ofelectrochemical degradation of corrugated steel pipe increases with lower pH andlower resistivity of soil and water. Reinforced concrete pipelines and culverts are alsovulnerable to electrochemical corrosion. Permeable to moisture, concrete may serve asthe electrolyte for highly anodic bare steel that can form where concrete cover hasspalled o� reinforcing bars. A potent corrosion cell may result.

Unlike metals, polyethylene pipes are non-conductors and are not vulnerable to galvanic corrosion associated with electrochemical attack. Polyethylene pipes are not degraded by pH extremes, aggressive salts or chemically induced corrosion.Unlike metals, HDPE pipes are non-conductors, insensitive to low soil resistivity,and therefore not subject to electrochemical corrosion. The Federal Lands Highway(FLH) policy is that plastic alternatives may be speci�ed without regard to resistivityand pH of the site. The same is true for many states.

HDPE pipes are e�ective for drainage of hostile e�uents, such as acid rain, acidicmine wastes, aggressive land�ll leachates and e�uents with high concentrations ofroad salts, fuels and motor oils. Laboratory studies indicate that only a negligibleincrease in abrasive wear of HDPE pipes may be expected when the pH drops fromneutral (pH = 7) to medium-low acidic conditions (pH = 4). A reported �eld studyshowed that HDPE pipe is una�ected by acid mine run-o� of pH ranging from 2.55 to 4.

Abrasion

Chemicals and abrasion are the most common durability concerns for drainage pipes,especially when the e�uent �ows at high velocities. In test after test, results show thatit takes longer to abrade through polyethylene than concrete and metallic pipes. Infact, in testing in both the United States and Europe, polyethylene has demonstratedwear rates up to 10 times less than steel.




Abrasives – such as stones or debris – can result in a mechanical wearing away of the pipe. The extent of the problem depends on the type of abrasive, frequency thatthe material is in the pipe, velocity of the �ow, and the type of the pipe material. The e�ect of abrasives may be seen in the pipe invert where exposure is most severe.Over time, abrasives can result in a loss of pipe strength or reduction in hydraulicquality as they gradually remove wall material.

Abrasion is a precursor to accelerated corrosion. The Federal Lands Highway Project Development Design Manual has de�ned measures of abrasion for typical�ow conditions (rather than a particular design �ood) as follows:

• nonabrasive – no bed load and very low velocities • low abrasive – minor bed loads of sand and velocities less than 1.5 m/s (5 fps) • moderate abrasive – moderate bed loads of sand and gravel and velocities between

1.5 and 4.5 m/s (5 and 15 fps) • severe abrasive – heavy bed loads of sand, gravel and rock and velocities exceeding

4.5 m/s (15 fps)

The FLH design guide permits unrestricted use of HDPE and PVC for nonabrasiveand low abrasive conditions. Many states permit the unrestricted use of plastic pipesfor all abrasive environments.

Other Durability Items

Ultraviolet (UV) radiation and oxygen induce degradation in plastics that usually alter the material’s physical and mechanical properties. The function of UV stabilizersis to inhibit the physical and chemical processes of UV-induced degradation. Themost common UV stabilizer used in the polyethylene pipe industry is �nely dividedcarbon black, which is the additive most e�ective in stopping these UV-induced reactions. However, colors with UV stabilizers, other than black, may be just ase�ective in inhibiting UV degradation.

The National Fire Protection Association (NFPA 704) rates polyethylene with a 1(slow burning) in a scale from 0 to 4; higher ratings indicate increasing vulnerability.Polyethylene piping, in sizes up to and including 18 in. (457 mm) diameter has beenused for 30 years in the natural gas industry without reported problems. Whereasprudence suggests that corrugated HDPE should be protected from exposure to majorgrass �res at drainage inlets and outlets, most states consider the risk insigni�cant orminimal. A Battelle study notes that the �ammability of plastic pipe is a non-issue.Non-HDPE pipes typically have linings and coatings used for protection against




corrosion and abrasion. Many of these coatings are also combustible. For all types ofpipes, if exposure to �re is a considerable risk, there are numerous preventive measuresthat can be considered to prevent �re damage. Rip-rap or gravel around exposed ends,steel end sections or other methods can be used to keep grass or combustibles awayfrom the pipe end.

Service Life

Control and disposal of surface water runo� during periods of abnormally high rainfall with associated �oods require e�cient and reliable systems of drainage of predictable longevity. Estimates of years of reliable low maintenance service, anticipated in the design phase, is dependent upon service experiences, choice of pipe materials, environmental considerations, regional construction practices and economic constraints.

The desired service life of a drainage system is speci�ed by the agency of jurisdiction.A 50-year design life is generally the minimum speci�ed; therefore a service life inexcess of that brings further economies to the installation. The service life of corrugatedHDPE pipe manufactured from today’s materials is expected to exceed 100 years.Well-de�ned and timely maintenance is key to achieving the anticipated longevity.Inspection strategies vary. Rehabilitation or replacement is justi�ed when it is unsafe,or uneconomical, to maintain elements of the drainage system in service. Trenchlessmethods of rehabilitating metal and concrete include sliplining, �exible tube liningand Portland cement mortar lining. The use of preformed linings of plastic are oftenfollowed with grouting of the annular space between the liner and the existing pipe.

Corrosion and abrasion damage to culverts and drainage pipelines is irreversible.Initial service life calculations must be inclusive of expectations of long-term durability, structural integrity and hydraulic capacity. When possible, useful servicelife may be extended by corrective measures. These costs must be weighed againstcosts of replacement. In cases of pipelines and culverts under high �lls, addressingassociated problems such as tra�c interruption may be very costly.

Life Cycle Cost Analysis

Comparisons of design alternatives often employ the use of life cycle economic analyses. The life cycle cost of an alternative system or part of a system anticipates all the costs that are likely to occur over the service life. Included are costs of the initial investment, inspection as well as scheduled maintenance, repair, rehabilitationand/or replacement and disruption of services. Estimates are required for useful




survival life, salvage credits, residual value, discount rate and time period of analysis.The likelihood of rural areas changing into urban areas and the associated needs forfuture increases in hydraulic capacity and accommodation of changes in aggressivenessof the e�uent must be incorporated into life cycle analyses.

Predictions of useful service lives for cross drains, side drains, storm drains, underdrains and sanitary sewers of all materials appear in a joint survey by the AmericanAssociation of State Highway and Transportation O�cials (AASHTO), the AssociatedGeneral Contractors (AGC) and the American Road and Transportation BuildersAssociation (ARTBA) . FLH policy requires all permanent drainage pipe installationsto be designed for a minimum of a 50-year maintenance-free service life – temporaryinstallations excepted.

Alternatives with di�erent costs are compared over the expected life of a project.Discount rates which include expectations of in�ation are estimated – a risky processwhich will signi�cantly in�uence the analysis. Low discount rates favor greater initialcosts and lower future expenditures and vice versa. The lowest present worth estimateof alternatives is the most sound economic basis for selection.

The present worth of a cost “n” years after the initial investment is obtained by multiplying a present worth factor (PWF) by the estimated expenditure. With “i”de�ned as the discount rate:

PWF = 1/(1+i) n Equation 7-1

Estimating a discount rate of 8%, what is the present worth of a $500,000 maintenance expenditure programmed to occur in 20 years? What is the presentworth of this same expenditure if the discount rate is estimated to be 10%?

i = 8%: PWF = 1/(1+i) n = 1/(1+.08)20 = 0.215Present Worth = $500,000(0.215) = $107,300

i = 10%: PWF = 1/(1+i) n = 1/(1+.10)20 = 0.149Present Worth = $500,000(0.149) = $74,300

Note the impact of the discount rate on the outcome, the value of which should be consistent with the economic policies of the organization being served. Also, the estimated cost of maintenance “n” years into the future is sensitive to presentapproximations of the course of in�ation.




Optional time periods for life cycle analyses include: desired service life, expected survival time to earliest rehabilitation or replacement, longest expectation of survival,time to anticipated capacity increase or any other period that is consistent with thephysical and economic constraints of the owner of the facility. Refer to the work ofT. J. Wonsiewicz (March 1990) for further discussion of discount rates and in�ation.

Uncertainty of any of the alternatives may seriously skew the outcome of life cycleanalyses. Experiences with pipes of di�erent materials in local environments similarto the drainage facility of interest should be a major in�uence in the assignment ofexpected survival life.




Bibliography

Cross-Reference for Drainage Pipe Speci�cations for Waterways, Airports, Railroads, Transit and Highways,AASHTO-AGC-ARTBA Joint Committee, Subcommittee on New Highway Materials, Task Force 22 Report.

Modern Sewer Design,Fourth Ed., American Iron and Steel Institute, 1990.

Cassidy, M., Review Speci�cations and Performance Requirements for Plastic PipeSystems,Battelle, Columbus, Ohio, 1994.

Economic Studies for Military Construction Design - Applications,TM 5-802-1, Department of the Army, Headquarters, December 31, 1986.

Gabriel, L.H., Abrasion Resistance of Polyethylene and Other Pipes,California State University, Sacramento, 1989.

Gabriel, L.H. and Moran, E.T., Service Life of Drainage Pipe, Synthesis of Highway Practice 254, Transportation Research Board, 1998.

Goddard, J.B., Nine Year Performance Review of a 24≤ Diameter Culvert in Ohio, Structural Performance of Flexible Pipes (Sargand, Mitchell & Hurd), Balkema, Rotterdam, 1990.

Kirschmer, O., Problems of Abrasion in Pipes, Steinzeugin Formationen, 1966, No. 1, pp. 3-13.

Federal Lands Highway Project Development Design Manual,U.S. Department of Transportation, Federal Highway Administration, FHWA-DF-88-003, June 1996 Metric Edition, section 7.4.D, (electronically published http://www.bts.gov/NTL/data/pddm-m.pdf).

Wonsiewicz, T.J., Life Cycle Cost Analysis, Discount Rates and In�ation,Pipeline Design and Installation, ed. K.K. Kienow, ASCE Pipeline Division, March 1990, pp. 639-648.



chapter 7: durability and service life - wikiplast

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